Plate buckling resistance patch loading of longitudinally stiffened webs and local buckling

Abstract: Incremental launching of steel bridges is a demanding undertaking, on the erection site as well as on the designers desk. Not seldom, the structure itself is during the launching subjected to high concentrated forces on the lower flange when passing over the launching shoe or a intermediate support (e.g. column). These concentrated forces, commonly referred to as patch loads, may be of such magnitude that it governs the thickness of the web in the bridge girder and a small increase in web thickness leads to a substantial gain of steel weight of the bridge. Hence also a higher material cost. One solution to this problem is to reduce the buckling length of the web with the use of a longitudinal stiffener of open (a plate) or closed type (closed profile of e.g. V-shape). The improved patch load resistance is in the Eurocode nowadays determined with the help of the yield resistance for the web and contributing parts of the loaded flange reduced with a factor dependent of the slenderness of the web and the influence of the longitudinal stiffener. Parts in the expressions for the yield resistance and the reduction factor have been somewhat questioned and over the years a substantial amount of tests and FE simulations of longitudinally stiffened webs has been carried out. This research work has produced a large amount of test data which has been used herein to further improve the prediction of the patch load resistance of longitudinally stiffened steel girder webs. Based on the use of the gathered test data from the literature and previously done research, a calibrated patch load resistance function was developed for both open and closed longitudinal stiffeners. Furthermore, a partial safety factor for the proposal was determined according to the guidelines in the Eurocode. In all, the proposal was proven to have an improved level of prediction than other comparable proposals (e.g. Eurocode). Another questioned part in the commonly used design codes is the reduction function concerning local buckling under uniformly in-plane compression. The nowadays used function (the Winter function) has been developed during the 30-ies based on tests on cold formed specimens and has been criticized as being too optimistic regarding plates with large welds. At LTU a series of tests on welded specimens made of high strength steel with large welds was conducted to investigate the aforementioned concerns. Along with test data found in a literature survey, the tests showed that the Winter function was shown to be too optimistic and a new reduction function, based on the test data, was proposed also for this application.